The present invention relates to the recovery of uranium from subterranean ore deposits and more particularly to in-situ leaching employing a heated carbonate lixiviant.
The various techniques for the production of uranium ore deposits may be characterized as falling within two general classes. One involves a surface milling operation in which uranium ore obtained by mining is crushed and blended and then subjected to a leaching procedure in which lixiviant containing a suitable leaching agent is employed to extract uranium from the milled ore. The uranium is then recovered from the pregnant lixiviant by a suitable technique such as solvent extraction, direct precipitation, or by absorption and elution employing an ion exchange resin. The other involves in-situ leaching in which a lixiviant is introduced into a subterranean ore deposit through a suitable injection system. The lixiviant solubilizes uranium values as it traverses the ore body. The pregnant lixiviant is then withdrawn from the ore body through a production system and treated to recover uranium therefrom by suitable techniques such as noted above. The lixiviants employed in either the aboveground mill leaching or the underground in-situ leaching may generally be classified as containing a carbonate or an acidic leaching agent. In acid leaching, the most commonly employed acid is sulfuric acid added in an amount to provide a pH in the lixiviant of about 2 or less. Carbonate lixiviants contain carbonate or bicarbonate ions or mixtures thereof which function to complex the uranium in the form of the water-soluble uranyl tricarbonate ion. The carbonate lixiviants may be formulated by the addition of alkali metal carbonates and/or bicarbonates or by the addition of carbon dioxide and an alkaline agent, e.g. sodium hydroxide, to control the pH. In most in-situ leaching operations, the lixiviant contains a carbonate leaching agent since the presence of carbonate materials in the subterranean ore deposits will lead to consumption of an acidic leaching agent and also raise the possibility of plugging of the formation due to precipitation of reaction produces such as calcium sulfate.
In uranium mills, it is a conventional practice to carry out the leaching operation at elevated temperatures. Thus, Merritt, Robert C., THE EXTRACTIVE METALLURGY OF URANIUM, Colorado School of Mines Research Institute, 1971, in pages 83-97, discloses the general concepts involved in milling operations employing a carbonate leaching agent and recognizes that the reaction rate varies with temperature. For example, Merritt, on page 87, states that each 10.degree. C. increase in temperature nearly doubles the reaction rate at constant pressure and, on page 94, states that typical operating temperatures for circuits using Pachuca tanks range from 75.degree. to 80.degree. C. Beverly et al., "Pilot Plant Alkaline Leaching of Uranium Ores", Proceedings of the Second United Nations International Conference on the Peaceful Uses of Atomic Energy, Vol. 3, United Nations, Geneva (1958), pp. 326-332, describe experimental work showing a similar relationship between temperature and reaction kinetics and note that although the rate of uranium dissolution varies markedly with temperature the amount of uranium ultimately extracted remains essentially the same.
Contrary to the practice followed in mill leaching operations, in-situ leaching operations have heretofore involved the injection of the lixiviant under ambient temperature conditions. Thus, U.S. Pat. No. 2,896,930 to Menke discloses an in-situ leaching process employing a carbonate leaching agent in a "cold" lixiviant, i.e. one injected at substantially atmospheric temperature or at the temperature of the underground deposit. Notwithstanding the teachings of Menke that "hot" leaching solutions are unnecessary, it has been proposed to precede the injection of lixiviant into a subterranean ore deposit by an in-situ combustion process. Thus, U.S. Pat. No. 2,954,218 to Dew et al. discloses carrying out an in-situ combustion process in a refractory ore deposit in which the uranium is associated with carbonaceous material which retards the leaching action of the lixiviant. In-situ combustion of the carbonaceous material is effected by the injection of air in order to heat the ore body to a temperature of about 600.degree. F. or more. By thus burning the carbonaceous material, the uranium is exposed to the action of the subsequently injected lixiviant thus facilitating the leaching procedure.